1. Field of the Invention
The present invention relates to the field of optical fibers and optical fiber manufacturing, and more particularly to optical fiber preforms which minimizing the diffusion of OH from the substrate tube to the core of an optical fiber.
2. Description of the Related Art
A single mode optical fiber is made by depositing a cladding layer and a core layer. In a DC-SM (depressed cladding-single mode) type, a cladding layer is deposited by doping SiO.sub.2 with P.sub.2 O.sub.5, GeO.sub.2, and F to lower the deposition temperature and the refractive index, a core layer for transmitting light is deposited by doping SiO.sup.2 with GeO.sub.2 to increase the refractive index, and then an optical fiber preform is manufactured through a collapsing and closing process.
In a process for manufacturing an optical fiber preform using modified chemical vapor deposition (MCVD), self-collapse of a substrate tube occurs during deposition as the deposition layer becomes thicker, resulting in an increase in the thickness of the tube. Also, a high temperature burner is required to sinter and consolidate a thick deposition layer, and the time for the collapsing and closing process becomes longer, so that a substrate tube becomes exposed to a high temperature over a long period of time.
In this process, when a very small amount of water (H.sub.2 O) (generally about several ppm) contained in the substrate tube diffuses into the deposition layer, diffused water is combined with P.sub.2 O.sub.5 or SiO.sub.2 deposited in the cladding region, thus forming P--O--H or Ge--O--H bonds. OH which diffuses to the core region is combined with SiO.sub.2 or GeO.sub.2 deposited in the core layer, thus forming Si--O--H or Ge--O--H bonds while dissolving Si--O or Ge--O bonding. O--H or P--O--H bonds formed in the combination with water in each deposition region as described above result in additional optical loss due to absorption bands at specific wavelength regions. In the case of a single mode optical fiber, wavelength bands in which serious optical loss occurs are the 124 .mu.m-1.385 .mu.m band due to the O--H bond combination, and the 1.2-1.8 .mu.m band due to the P--O--H bond combination. When OH is diffused into the core region, it forms a non-bridging oxygen (NBO), and the structural homogeneity of glass material of the core layer is thus locally deteriorated, which causes density fluctuation of the core layer. Consequently, scattering loss is increased.
The inside and outside diameters of a tube contract with an increase in the thickness of the deposition layer during sintering performed simultaneously with deposition, so that it is difficult to obtain an appropriate diameter ratio (that is, cladding diameter/core diameter=D/d). Therefore, a distance sufficient to prevent diffusion of OH cannot be secured, thus greatly increasing loss due to OH.
In the prior art, a method of thickening the cladding layer is used to prevent OH from diffusing from the substrate tube to the core layer. However, when a large-aperture preform is manufactured by this method, contraction of the tube makes it difficult to secure an appropriate diameter ratio, and a burner of a higher temperature is required during deposition of the core layer since the efficiency of transmitting heat to a core layer is degraded due to an increase in the thickness of the tube layer. Thus, the tube is exposed to high temperature for a long time, thus increasing loss due to OH.